1. Field of the Invention
The present invention relates to a semiconductor laser light source tunable in wavelength, which is used in any technical field which requires a light signal source, in particular, in an optical telecommunication or a coherent optical measuring technical field.
2. Description of Related Art
In the coherent optical measuring technical field, an external resonator type of semiconductor laser (hereinafter, may be simply referred to LD) is generally used as a light source unit in a light source tunable in wavelength. An optical filter which is a wavelength selection element is disposed in the external resonator thereof to get a singlemode oscillation.
Such a technique enables wavelength scanning in a wide range by mechanically varying the transmitted (or reflected) wavelength from the optical filter.
FIG. 11 shows a construction example of a conventional wavelength tunable LD light source. In this figure, the reference numeral 1 denotes an external resonator type of LD light source unit, 2 denotes an optical filter, 3 denotes a drive unit, 4 denotes a control unit, and 20 denotes an origin switch.
In the wavelength tunable LD light source in FIG. 11, first, when a power source is turned on, the drive unit 3 is moved to a position at which the origin switch 20 is operated thereby, by the control unit 4. When the origin switch 20 is operated, the drive unit 3 is reset and the position thereof is used as the origin.
The wavelength of the output of the wavelength tunable LD light source when the drive unit 3 is at the origin is measured in advance by using a precise wavemeter and is memorized as the origin wavelength.
The drive unit 3 can mechanically change wavelength of the transmitted beam or of the reflected beam, from the optical filter 2, and when occasion demands, it carries out also adjustment for the length of an external resonator.
The relationship between the state of the drive unit 3 and the oscillation wavelength of the external resonator is already known. In the control unit 4, a wavelength set is carried out on the basis of a formula showing the known relationship. The control unit 4 also controls the LD drive current in order to control the optical output level of the LD light source unit 1.
FIG. 5 shows a construction example of an external resonator type of LD light source unit 1. In FIG 5, the reference numeral 101 denotes a diffraction grating; 102, 105 and 107 denote lenses; 103 denotes an anti-reflection film; 104 denotes an LD; 106 denotes an optical isolator; 108 denotes an optical fiber; and 109 denotes an LD driving circuit.
In the external resonator type of LD light source unit 1, shown in FIG. 5, the diffraction grating 101 which corresponds to the above-described optical filter 2, as shown in FIG. 4, also functions as a mirror in one side of the external resonator.
That is, the external resonator is formed by the end surface B of the LD and the diffraction grating 101. When denoting the intersection of the surface of the diffraction grating 101 and the optical axis X with A, the length of the resonator is defined by the segment AB of a line.
An anti-reflection film 103 is formed on an end surface in the side of the diffraction grating 101, of the LD 104 in order to remove unnecessary reflection.
Each of the lenses 102 and 105 is a collimator for changing the output beam of the LD 104 to a collimated beam.
The output beam from the external resonator LD 104, which is obtained from the side of the LD end surface B, is condensed through the lens 107 and is taken out by the optical fiber 108.
In order to generate no noise due to external feedback beam from a following optical system, the optical isolator 106 is disposed on the way in the output side.
The LD driving circuit 109 feeds an LD driving current corresponding to a desired optical output level.
Although only the diffraction grating 101 is shown in FIG. 5, as the optical filter 2, it is possible to use an optical element other than the diffraction grating 101, for example, to use an interference filter or the like, as the optical filter 2. When occasion demands, combination of a plurality of optical elements can be also used as the optical filter 2.
Next, the characteristics of an optical filter will be explained as follows.
FIG. 6 shows an optical system of a diffraction grating 101. In this FIG., .theta. is the angle of the normal N.sub.gr to the diffraction grating 101 with the optical axis, d is the pitch of the diffraction grating, and the incident light and the reflected (diffracted) light are set on the same optical axis X, like the above-described external resonator shown in FIG. 5.
The spectrum of the reflected light when an incident white light came into the diffraction grating 101 is the filter characteristics of the diffraction grating. A filter characteristics is obtained, as shown in FIG. 7.
The reflection peak wavelength .lambda..sub.gr is given by the following Bragg's formula: EQU .lambda..sub.gr =2d.times.sin (.theta.) (1)
The characteristics of the interference filter 201 in the optical system shown in FIG. 8 has periodic transmittance peaks, as shown in FIG. 9.
In FIG. 8, "D" is the thickness of the interference filter 201, and "n" is the refractive index thereof.
The wavelength of each transmittance peak is given by the following formula : EQU k.lambda..sub.k =2nD.times.cos (.theta.) (2)
where .theta. is the angle of the normal N to the interference filter 201 with the optical axis in the interference filter, and k is an integer.
When L is the length of the external resonator and m is an integer, like the case of an interference filter, the oscillation longitudinal mode of the external resonator is expressed as follows: EQU m.lambda..sub.m =2L (3)
Next, an example of characteristics of wavelength tunable LD light source, using an optical filter comprising a combination of a diffraction grating and an interference filter is illustrated in FIGS. 10A to 10E.
FIG. 10A shows a gain characteristics of LD, which generally has a gain in a wavelength range not less than 100 nm.
FIG. 10B shows a resonator mode corresponding to the formula (3), which are oscillation longitudinal modes.
Several modes are selected by using the filter characteristics of the diffraction grating shown in FIG. 10C, among the oscillation modes.
Then, a single mode is selected by using the filter characteristics of the interference filter shown in FIG. 10D. As a result, a single mode oscillation is obtained, as shown in FIG. 10E.
By changing each of the characteristics shown in FIGS. 10B, 10C, and 10D, that is, by changing L, .theta., and .phi. suitably, it is possible to carry out wavelength scanning.
The relationship between the wavelength and L, .theta., and .phi. is found on the basis of previous measurements. According to the relationship, the drive unit 3 having a combination of a motor, a rotary table, a directly linear-moving mechanism and the like realizes a state of particular values of L, .theta., and .phi., corresponding to the set wavelength.
The so-called WDM (Wavelength division multiplexing) of optical communication system which is recently focused on is one multiplexing several wavelengths with a difference of wavelengths of about 1 nm. For example, when the wavelengths difference is finely adjusted at a level of about 0.1 nm, wavelength accuracy having a level of about 0.01 nm which is taken a figure down in comparison with that of the wavelengths difference is required.
On the contrary, although the obtained set resolving power in the former art is a level of 0.001 nm, the set wavelength accuracy thereof is about a level of .+-.0.1 nm because of error factors, e.g., a backlash or a hysteresis on the mechanism, or set condition reproducibility including a fluctuation in temperature, a change with the passage of time or the like.
Therefore, in order to improve the set wavelength accuracy, it was required to prepare not only an external resonator type of wavelength tunable LD light source but also an expensive wavemeter, to measure the wavelength of the light source output by the wavemeter, and to correct the setting.